DELTA NATIONAL WILDLIFE REFUGE, LA. - At the southernmost fringe of the Mississippi River, a home base for migratory birds, wildlife managers are bracing for the worst.

Roughly 419,000 gallons of fuel oil were dumped into the river after a collision early Wednesday between a tanker and a barge in New Orleans.

Cleanup crews farther upriver are redirecting oil to shorelines fortified by the levee system, where the muck can then be scrubbed, absorbed and trucked away.

Attempts to place the booms used to trap the oil have been chancy because of the fast currents in wide, deep navigation channels near the mouth of the river. That water will flow into adjacent marshes and eventually into the gulf.

In the shallower and narrower waterways near Delta National Wildlife Refuge, crews are setting up hundreds of feet of barriers to prevent oil from killing marsh plants that provide food to nearly 100,000 migratory birds every fall.

"This is a major wintering area for waterfowl," said James Harris, a senior wildlife biologist at the refuge.

The main concern for biologists so far is the plants, not the animals. Most of the avian traffic at the refuge comes in the fall. If the plants die from the oil, it could mean a full year before they regenerate.

So far cleanup crews hired by American Commercial Lines, the owner of the tugboat and the barge involved in the collision, had laid out nearly 13 miles of the floating barriers meant to catch the oil.

Five separate cleanup contractors are working up and down the river, skimming oil from the surface and scrubbing the banks with industrial cleanup brushes, sometimes even shovels. The cleanup is expected to take weeks.

Meanwhile, ships began crawling up the Mississippi at New Orleans in a tightly controlled procession Friday, two days after a massive oil spill shut down a stretch of one of the nation's most critical commercial arteries.

With more than 200 ships to be cleared, it was expected to take days to clear the backlog.

The shutdown of a 100-mile stretch of river halted vessels ranging from oil supertankers to grain barges in one of the world's busiest ports. \Gary LaGrange, executive director of the Port of New Orleans, said a recent economic impact study conducted by the port showed that such a total shutdown could cost the national economy up to $275 million per day.

A cleaning station was set up near the river's mouth to scrub the hulls of vessels on their way to the gulf. Another was being set up on the river near the New Orleans suburb of Westwego. Officials said high-pressure water would be applied to the ships' hulls at the water line to remove oil -- a process that would take three to four hours per ship.

The Associated Press contributed to this report.